1. Field of the Invention
The present invention relates to a method of improving the gradation, contrast and resolution of images and pictures obtained by electrophotography.
2. Description of the Prior Art
In conventional, electrophotographic copying machines, the entire cycle of a series of steps and operations performed in order to generate an electrophotographic reproduction of an image or picture, including the steps of exposure, development and transfer, occurs during one rotation cycle of an electrophotographic drum which has a photoreceptor surface layer to capture the image or picture being reproduced. Accordingly, the print density of the electrophotographic reproduction or copy obtained using conventional electrophotographic copying machines and the print density of the original image or document may be compared to determine the effectiveness and accuracy of the electrophotographic copying machine. In FIG. 6, solid line A illustrates the relationship of the print density of a copy as a non-linear function of the print density of the original document in conventional, electrophotographic copying machines. Solid line A illustrates the characteristic initial steep slope at the lower print density region and the flattening slope of saturation at the higher print density region. The relationship exhibited by solid line A is caused by various factors such as the developing agent and the sensitivity of the photoreceptor layer on the electrophotographic drum. In most instances, a document to be electrophotographically reproduced comprises only text and/or simple outline graphics and therefore has only two print densities--black and white, and the characteristic non-linear relationship (as shown by line A in FIG. 6) is not a serious problem.
In the case of documents such as photographs which have varying gradation and shading and changing print densities which merge in continuous fashion, the characteristic non-linear relationship (as shown by line A in FIG. 6) negatively impacts and deteriorates the gradation, contrast, resolution, print density and general quality of the reproduced copy relative to the gradation, contrast, resolution, print density and general quality of the original document. Accordingly, in reproducing documents having varying gradation and shading with changing print densities such as photographs, it is desirable to achieve a reproduction copy print density which is directly proportional to the print density of the original document. The relationship between the print density of the reproduction copy and the print density of the original document is, therefore, preferably a linear function as illustrated by the dotted line B in FIG. 6.
Previous attempts to achieve improved gradation, contrast and resolution in electrophotographically reproduced images may be seen in U.S. Pat. No. 2,868,642; in Japanese Patent Publication No. 48-17335; and in an article by Suzuki in the "Journal of
Electrophotographic Society, " Vol. 25, No. 1, pp. 52-58 (1986). These prior art methods employ electrophotographic copying methods in which the various parameters affecting the formation of a latent image are altered and modified in relationship with changes in print density of the original document to produce multiple latent images which represent different print densities of the original document. The latent images thus obtained are then developed and transferred onto the recording media in a superimposed manner to generate a reproduction copy of the original document.
In these previous attempts, a first step comprises forming a latent image under conditions optimal for the reproduction of lower print density portions of the original document, and thereafter transferring the image onto paper to reproduce the lower density portions of the original document. The second step comprises forming latent images under conditions optimal for the reproduction of higher print density portions of the original document, and thereafter successively transferring the latent image in a superimposed manner onto the same paper in order to reproduce the original document with relatively high gradation and contrast By "higher print density," reference is made to increased developing bias for a higher exposure parameter to the photoreceptor drum so as to produce a latent image comprising only the high print density portion of the original document. Similarly, by the term "lower density," reference is made to a low developing bias for a lower exposure factor to the photoreceptor drum so as to produce a latent image comprising a lower print density portion of the original document including a portion of the higher print density portions as well.
The above-mentioned prior art methods, however, suffer from a common problem in that it is difficult to obtain a desired print gradation and contrast since an image of a lower print density portion is first formed after which higher print density images are formed in superimposed fashion. The problem of this approach will be described with reference to FIGS. 7(a)-7(e). FIG. 7(a) is a schematic diagram in which the photoreceptor 20, toner, paper P and a transfer drum 32 are shown. In normal use, although the photoreceptor 20 and the developing drum 32 are in contact with each other with paper P interposed between them, for the purpose of facilitating explanation and description, the photoreceptor 20 and the developing drum 32 are shown apart from each other with the paper P on the developing drum 32.
Referring to FIGS. 7(a)-7(e), if lower print density portion reproduction is performed first, an image T.sub.L of the lower print density portion is formed on the photoreceptor 20 as shown in FIG. 7(a). This image T.sub.L is transferred by means of a transfer electric field onto paper P disposed on a transfer drum 32, as shown in FIG. 7(b). Next, a higher print density portion reproduction is performed to form a latent image T.sub.H of the higher print density portion on the photoreceptor 20 as shown in FIG. 7(c). The image or picture area of the image T.sub.L of the lower print density portion is larger than that of the image T.sub.L of the higher density portion. This is due to the fact that while the image of the entire, original document from the higher print density portions (highlight portion) to the lower print density portions (shadow portion) is continuously formed during lower print density portion reproduction, only an image with print density higher than a predetermined print density is reproduced in the higher print density portion reproduction.
No problem is presented if the image T.sub.H of the higher print density portion is transferred perfectly over the image T.sub.L of the lower print density portion in a superimposed manner as shown in FIG. 7(d). However, as in the usual case, a considerable quantity of toner in the portion of a lower print density portion image area, except the higher print density portion image area being transferred, is transferred back from the paper P to the photoreceptor 20 due to a so-called retransfer of toner as shown in FIG. 7(e). The problem which results is that the lower print density portion reproduction is not performed satisfactorily.
The reason why such retransfer of toner occurs may be explained with reference to FIG. 8. FIG. 8 is a schematic diagram illustrating for representation purpose only the toner retransfer phenomenon.
In FIG. 8, toner T adhering onto the photoreceptor 20 through development is transferred to paper P which is adsorbed on a transfer drum 32. Although the toner T (assumed to be negatively charged) sandwiched between the paper P and the photoreceptor 20 receives a sufficient transfer of electrical force to transfer the toner toward the paper P, the toner on the surface of the photoreceptor 20 suffers from a counter force which prevents transfer of the toner from the surface of the photoreceptor 20 to the paper P due to an adhesion force produced on the surface of the photoreceptor 20--mainly mirror forces and Van der Waals forces. Thus, in the usual transfer of toner, if there is a thick layer or a large quantity of toner T on the surface of the photoreceptor 20, the toner T is in a stacked configuration on the photoreceptor 20 as shown in FIG. 8, and most of the toner T (80% to 90%) is transferred to the paper P while toner Ta in direct contact with the surface of the photoreceptor 20 is not transferred. However, if there is a thin layer or only a small amount of toner on the photoreceptor 20, most of the toner is in direct contact with the surface of the photoreceptor 20. As previously noted, since the toner in contact with the surface of the photoreceptor 20 is not easily transferred onto the paper P due to the adhesive forces produced on the surface of the photoreceptor 20, the amount of toner transferred, that is, the transfer efficiency, is lowered to 30% to 40%, as shown in FIG. 9.
In multiple transfer, therefore, if lower print density portion reproduction is performed first, toner adhering to paper P is sandwiched between the paper P and the photoreceptor 20 again, so that the toner is transferred to the photoreceptor 20 and thereby causes the toner retransfer problem.
Based upon the toner retransfer problem, the relationship between the print density of an image obtained in practice by composition of images using multiple transfer and the print density of the original is illustrated by the solid line C in FIG. 10. The characteristics of line C is different from the characteristics of line D added on calculation as shown by the broken line. The ideal density characteristic is shown as dotted line E. It is apparent from FIG. 10 that previous attempts at improving the gradation and quality of images by multiple transfer of latent images has not altogether been successful.
Since the amount of toner transferred by retransfer varies erratically with changes in environmental operating conditions, an additional problem exists in that the desired picture quality cannot be obtained through superimposition of the latent images even if adjustments in lower and higher print density portions are independently made relative to each other, and therefore the difficulties associated with adjusting both the lower and higher print density portions are appreciated.